Implantable medical device with temperature measuring and storing capability

ABSTRACT

An implantable medical device such as a cardiac pacemaker or implantable cardioverter/defibrillator with the capability of storing body temperature measurements taken periodically and/or when triggered by particular events.

FIELD OF THE INVENTION

[0001] This invention pertains to implantable medical devices such ascardiac rhythm management devices. In particular, the invention relatesto a method and system incorporated into such a device for gatheringclinically useful physiological data.

BACKGROUND

[0002] Implantable cardiac rhythm management devices are commonplacetoday for the treatment of chronic or recurring cardiac arrhythmias. Forexample, cardiac pacemakers are implantable devices that replace orsupplement a heart's compromised ability to pace itself (i.e.,bradycardia) due to chronotropic incompetence or a conduction systemdefect by delivering electrical pacing pulses to the heart. Implantablecardioverter/defibrillators (ICD's) are devices that deliver electricalenergy to the heart in order to reverse excessively rapid heart rates(tachycardia) including life threatening cardiac arrhythmias such asventricular fibrillation. Since some patients have conditions thatnecessitate pacing and also render them vulnerable to life-threateningarrhythmias, implantable cardiac devices have been developed thatcombine both functions in a single device.

[0003] Cardiac rhythm management devices are typically implantedsubcutaneously or submuscularly in a patient's chest and have leadsthreaded intravenously into the heart to connect the device toelectrodes used for sensing and pacing. Leads may also be positioned onthe epicardium by various means. A programmable electronic controllercauses shocks to be delivered when fibrillation is detected or pacingpulses to be output in response to lapsed time intervals and sensedelectrical activity. Pacemakers sense intrinsic cardiac electricalactivity by means of internal electrodes disposed near the chamber to besensed.

[0004] Modern cardiac rhythm management devices also typically have thecapability to communicate data via a data link with an externalprogramming device. Such data is transmitted to the pacemaker in orderto program its mode of operation as well as define other operatingparameters. Data transmitted from the pacemaker can be used to verifythe operating parameters as well as relay information regarding thecondition of both the pacemaker and the patient. Pacemaker patients aremonitored at regular intervals as part of routine patient care and tocheck the condition of the device. Among the data which may typically betelemetered from the pacemaker are its programming parameters and anelectrogram representing the electrical activity of the heart as sensedby the pacemaker.

[0005] Pacemakers have also been developed which monitor a patient'sexertion level while the device is functioning in order to adjust thepacing rate. Such devices, referred to as rate-adaptive pacemakers, mayuse various measurable physiological parameters for this purpose thatare related to exertion level including minute ventilation, bodyactivity, electrogram intervals, and body temperature. Because of theircontinuous access to the patient and their communications capabilities,cardiac rhythm management devices and other similar implantable devicesmay also offer an ideal platform for gathering and storing clinicallyuseful information which can later be transmitted to an external device.

SUMMARY OF THE INVENTION

[0006] The present invention is a method and system for monitoringtemperature in an implantable medical device such as a cardiac pacemakeror implantable cardioverter/defibrillator. Data from these monitoringoperations can then be stored in the memory of the device for laterretrieval using an external programmer. In one embodiment, temperaturemeasurements are collected at specified regular intervals to ascertaintrends in the patient's temperature that may be useful in diagnosingcertain medical conditions. The collection intervals may be made shorteror longer in order to detect fast or slow trends, respectively.Temperature data may also be collected to reflect average temperaturevariations over specified periods (e.g., daily) and at specific times ofthe day. Averages of temperature measurements taken over a period oftime can also be used to compensate for component drift in thetemperature sensor by calibrating the temperature sensor to match anaverage temperature which is assumed to be consistently maintained bythe body.

[0007] In another embodiment, temperature measurements are associatedwith contemporaneous physiological measurements such as heart rate,respiratory rate, and body activity as well as any device activity toaid the clinician in interpreting the data. Detection of certain eventssuch as arrhythmias or initiation of particular device activities may beused to trigger measurement of temperature and possibly otherphysiological variables.

[0008] Body temperature measurements may be taken using a temperaturesensor incorporated into an intravenous lead or otherwise external tothe device. Alternatively, temperature sensing circuitry internal to thedevice housing may be employed to provide the temperature measurement.Because device activity can affect its internal temperature, the systemmay require that temperature measurements using an internal sensor onlybe taken during periods in which the device is not actively deliveringtherapy. In a particular embodiment of an internal sensor, aproportional-to-absolute-temperature (PTAT) current typically used bythe device electronics to generate a reference voltage is employed as atemperature sensor. Incorporating a temperature sensor within the devicehousing allows a clinician to determine if the temperature is within anacceptable range before allowing the device to become operational in apatient and also allows monitoring of temperature before implantation todetermine if temperature extremes have occurred during storage which mayadversely affect device operation. As the internal temperature of anelectronic device may change during certain activities, such as deliveryof shock pulses in the case of an implantablecardioverter/defibrillator, either no temperature data is collectedduring such activities or such data is flagged accordingly.

[0009] Another embodiment of the invention involves efficient storageand transmission of temperature data. Because of the limited range oftemperature measurements taken within the human body, the range andresolution of stored temperature data can be adjusted so that lessstorage space is needed and data can be transmitted more efficiently. Anon-linear range can also be used so that different resolutions are usedwith different temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a system diagram of an implantable medical deviceincorporating the invention.

[0011]FIG. 2 is a block diagram of a particular implementation of aninternal temperature sensor.

DETAIL DESCRIPTION

[0012]FIG. 1 shows a system diagram of an implantable medical device, inthis case is a microprocessor-based pacemaker with defibrillation and/orantitachycardia pacing capability that incorporates the presentinvention. A microprocessor controller 10 communicates with a systemmemory 12 via a bidirectional system bus. Memory 12 may typicallycomprise a ROM for program storage and a RAM for data storage. Theoverall operation of the device is controlled by a system programrunning from the memory 12. The microprocessor also has a port forcommunicating with the telemetry interface 40 which in turn receivesprogramming data from and transmits telemetry data to an externalprogrammer 70 by a radio or other data link. The pacemaker has atrialsensing and pacing channels comprising electrode 34, lead 33, sensingamplifier 31, pulse generator 32, and an atrial channel interface 30which communicates bidirectionally with a port of microprocessor 10. Theventricular sensing and pacing channels similarly comprise electrode 24,lead 23, sensing amplifier 21, pulse generator 22, and a ventricularchannel interface 20. For each channel, the same lead and electrode areused for both sensing and pacing. The channel interfaces includesampling circuitry and an analog-to-digital converter for digitizingsensing signal outputs from the sensing amplifiers and registers whichcan be written to by the microprocessor in order to control pacing. Anexertion level sensor 90 is also provided for rate-adaptive pacing. Theexertion level sensor may measure, for example, respiratory rate, minuteventilation, or body activity with an accelerometer. A shock pulsegenerator 80 is also interfaced to the microprocessor for deliveringdefibrillation pulses to the heart via a separate pair of electrodes 81a and 8 lb.

[0013] A temperature sensor 50 communicates with the microprocessor viaa sensor interface 51. The sensor 50 may be a resistive temperaturedetector driven by a current source that converts temperature changes inthe patient's body into electrical signals. The sensor 50 may beincorporated into an external lead (e.g., an intravenous lead) or may beinternal to the housing of the device. An example of such an internalsensor is described more fully below. The sensor interface 51 includessampling circuitry for sampling the sensor output and ananalog-to-digital converter for digitizing the samples that are thenprocessed and stored by the microprocessor.

[0014] In accordance with the invention, the controller 10 is configuredto store temperature measurements collected at specified regularintervals and/or collected when triggered by sensed events or initiationof particular device activity. For example, temperature measurements maybe collected and stored at specified times on a daily basis with anassociated time stamp. Other physiological measurements may also besimultaneously collected and associated with a temperature measurement,including measurements of heart rate, respiratory rate, minuteventilation, or body activity. A temperature measurement may also betriggered by other sensor activity such as when a measured exertionlevel measurement reaches a specified limit value or upon detection of aspecified cardiac rhythm.

[0015] One use of periodic temperature measurements is to calculate atrend from the measurements indicating temperature changes over aspecified period of time. The device can also be programmed to enabletrending for a combination of heart rate, accelerometer measurements,respiratory rate, temperature, or other such inputs. Besides varyingwhich inputs to trend, and the trending rate (e.g. fast/slow) can alsobe selectively varied, and each of the trended inputs could be sampledat an independent rate. For example, the heart rate could be sampledonce every 16 seconds while the temperature is only sampled once every15 minutes. The fast trending rate for the temperature could be onceevery 15 minutes and the slow trending rate could be once every hour.This trending data could then be read from the device on a daily orweekly interval depending on the trending rate. Another option is togather trending information around some critical point in time. Forexample trending data could be gathered only around cardiac events oraround patient activated times. Even if trending is not programmed, thebasal temperature can still be recorded on a daily basis. This can bedone by measuring the temperature at set time (e.g. 3 am) or after thepatient activity is at a minimum (e.g. 2 hours at lower rate limit).This basal temperature data could be kept for the last 30 days or otherspecified time period.

[0016] Temperature sensors can drift over time due to component drift orsuch things as flicker noise. For example, when a PG is first calibratedit may read 98.8 deg as 98.7 degrees, and over time the error will vary.Over the life of the product the temperature error could be as much as afew degrees. As long as the error is known it can be subtracted from theindicated measurement in order to find the actual temperature. Onemethod to do this automatically is to take advantage of the consistentaverage temperature of the human body. For example, if over a week theaverage temperature measured is 97.0 degrees, it could be assumed thatthe error term is −1.8 degrees. All the temperature measurements for theperiod can then be scaled up by 1.8 degrees. The temperature sensor canalso be calibrated by programming the device with an actual temperaturewhen measured by other means.

[0017] Temperature data collected as described above may be transmittedvia the telemetry data link to an external programmer. After thetemperature data has been transmitted, it can be further processed andgraphically displayed. The further processing could include any of thefollowing: adjusting temperature data to account for drift error in themeasurement (i.e. autocalibration), comparing the temperature data toother previous data to determine trends, combining temperature data withdevice activity or other sensors, and plotting temperature verses adaily cycle or monthly cycle.

[0018] The temperature sensor 50 may either be incorporated into anintravenous lead or located within a housing for the sensor. The sensormay be of any convenient type such as a thermistor, resistivetemperature detector, or thermocouple. A particular embodiment of atemperature sensing circuit internal to the device housing utilizes theproportional-toabsolute-temperature (PTAT) current typically generatedby the device electronics. A PTAT current is normally used to generate areference voltage with a bandgap reference voltage circuit, but alsoprovides a convenient way of measuring the device temperature. Exceptwhen heat is being generated by the device, the device temperature isequilibrated with the body temperature so that the PTAT current varieswith body temperature. FIG. 2 is a block diagram of a possibleimplementation of such a temperature sensor. A PTAT current source 100feeds into an oscillator 101 that generates a clock signal with afrequency proportional to the IPTAT current. A counter 102 compares theoscillator clock frequency to a stable timebase such as could begenerated by a crystal oscillator 103. The data out of the counter 102is then a number that is proportional to temperature that is processedby circuitry 104 and which can be transmitted to an external programmerdisplay 106. As described above, the temperature data may also beprocessed with signals from other sensors 105.

[0019] Incorporating the temperature sensor within the device housingmeans that the sensor is subject to heating caused by, for example, highcurrents when the device is delivering shock therapy or reforming theelectrolytic capacitors used to deliver shock therapy. Temperaturemeasurements may therefore be prohibited from being collected duringsuch activity or within a specified time window afterward.Alternatively, such temperature measurements may be flagged accordingly.

[0020] Having a temperature sensor incorporated into the device housingalso allows monitoring of temperatures before implantation such as whenthe device is being stored for long periods of time. During storage ofthe device, for example, the temperature may be measured once per hourwith an alarm flag set if the temperature ever leaves safe storagetemperature limits. The flag can be announced whenever the device isinterrogated. Minimum and maximum storage temperatures can also belogged. The device can also be configured to issue an alarm if thepresent device temperature is not inside the safe operationaltemperature limits. This can happen because the storage temperaturelimits are broader than the operational temperature limits. If forexample, the device has been brought in from a very cold environment(such as outside winter temperatures) and has not had sufficient time towarm up, the device could be outside of the operational temperaturelimits but still within safe storage limits.

[0021] Another aspect of the invention involves the manner in whichtemperature data is represented which impacts both the required storagespace and transmission bandwidth. One method of compressing the storagespace and transmission bandwidth of temperature data is to assume afixed offset or to use a nonlinear compression scheme. For example, ifan 8 bit linear scale is used to store temperature data, then the scalecould be from 0 to 127.5 degrees with 0.5 degree resolution which is toocoarse a resolution. With 0.1 degree resolution, the temperature rangewould only be 0.0 to 25.5 degrees, which is too small. If a 90 degreeoffset were to be used, the temperature range would become 90.0 to 115.5degrees with a 0.1 degree resolution. This means that the 8 bit numberrepresents the difference between 90 degrees and the actual temperaturereading (e.g. a temperature of 98 degrees would be represented as 180,so 90+180/10=98). This would yield good temperature resolution over alimited range. The range and resolution could be adjusted for recordingdifferent types of information (e.g. −40 degree offset, 1 degreeresolution as a coarse temperature range). An example of a nonlinearrange would be to use a different resolution depending on thetemperature. For example temperatures between −40 to 90 and 116 to 178could have resolutions of 2 degree, and temperatures between 90 and115.4 could have resolutions of 0.2 degrees.

[0022] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A medical device adapted for implantation into apatient, comprising: a temperature sensor for measuring a temperaturewithin the patient's body and generating temperature signals inaccordance therewith; sampling circuitry and an analog-to-digitalconverter for producing digitized samples of a signal received from thetemperature sensor; a controller for processing and storing temperaturedata derived from the temperature signals; wherein the controller isconfigured to store temperature measurements collected at specifiedregular intervals and calculate a trend from the measurements indicatingtemperature changes over a specified period of time.
 2. The device ofclaim 1 wherein the controller is further configured to storetemperature measurements as specified times on a daily basis with anassociated time stamp.
 3. The device of claim 1 wherein the controlleris further configured to calibrate the temperature sensor by adjustingsensor measurements by an amount equal to the difference between anaverage temperature measurement and a nominal temperature assumed to bemaintained by the patient's body.
 4. The device of claim 1 furthercomprising a sensing channel for sensing cardiac electrical activity andwherein the controller is configured to associate a temperaturemeasurement with a simultaneously measured heart rate.
 5. The device ofclaim 1 further comprising an exertion level sensor and wherein thecontroller is configured to associate a temperature measurement with asimultaneously measured exertion level.
 6. The device of claim 5 whereinthe exertion level sensor is selected from a minute ventilation sensor,respiratory rate sensor, and a body activity sensor.
 7. The device ofclaim 5 wherein the controller is configured to trigger a temperaturemeasurement when a measured exertion level measurement reaches aspecified limit value.
 8. The device of claim 4 wherein the controlleris configured to trigger a temperature measurement upon detection of aspecified cardiac rhythm.
 9. The device of claim 1 wherein thecontroller is configured to store the temperature data with a variableresolution depending upon the temperature where the resolutiondetermines the range of temperatures represented by a particular numberof bits.
 10. The device of claim 1 wherein the temperature sensor isincorporated into an external lead.
 11. The device of claim 1 whereinthe temperature sensor is located within a housing for the device.
 12. Amedical device adapted for implantation into a patient, comprising: atemperature sensor for measuring a temperature within the patient's bodyand generating temperature signals in accordance therewith; samplingcircuitry and an analog-to-digital converter for producing digitizedsamples of a signal received from the temperature sensor; a controllerfor processing and storing temperature data derived from the temperaturesignals; wherein the temperature sensor is located within a housing forthe device.
 13. The device of claim 12 wherein the temperature sensorutilizes a proportional-to-absolute-temperature current source togenerate a temperature signal.
 14. The device of claim 12 wherein thedevice is a cardiac rhythm management device having at least one sensingchannel for sensing cardiac electrical activity.
 15. The device of claim14 wherein the controller is further configured to calibrate thetemperature sensor by adjusting sensor measurements by an amount equalto the difference between an average temperature measurement and anominal temperature assumed to be maintained by the patient's body. 16.The device of claim 14 wherein the controller is configured to associatea temperature measurement with a simultaneously measured heart rate. 17.The device of claim 14 further comprising an exertion level sensor andwherein the controller is configured to associate a temperaturemeasurement with a simultaneously measured exertion level.
 18. Thedevice of claim 14 wherein the controller is configured to not gathertemperature measurements during periods when heat is being generated bythe device.
 19. The device of claim 14 wherein the controller isconfigured to flag temperature measurements taken during periods whenheat is being generated by the device.
 20. The device of claim 14wherein the controller is configured to set an alarm if the device'stemperature is not within operational limits.
 21. A method for operatingan implantable medical device, comprising: measuring a temperaturewithin a patient's body with a temperature sensor and generatingtemperature signals in accordance therewith; producing digitized samplesof a signal received from the temperature sensor and processing andstoring temperature data derived from the temperature signals; and,storing temperature measurements collected at specified regularintervals and calculating a trend from the measurements indicatingtemperature changes over a specified period of time.
 22. The method ofclaim 21 further comprising storing temperature measurements asspecified times on a daily basis with an associated time stamp.
 23. Themethod of claim 21 further comprising calibrating the temperature sensorby adjusting sensor measurements by an amount equal to the differencebetween an average temperature measurement and a nominal temperatureassumed to be maintained by the patient's body.
 24. The method of claim21 further comprising sensing cardiac electrical activity andassociating a temperature measurement with a simultaneously measuredheart rate.
 25. The method of claim 21 further comprising transmittingtemperature measurements to an external programmer.
 26. The method ofclaim 20 further comprising measuring an exertion level sensor andassociating a temperature measurement with a simultaneously measuredexertion level.